An investigation of the prevalence, diagnosis, and mechanisms of resistance to antiviral agents in herpes simplex virus (HSV) will be undertaken. Advanced stages of immunosuppression, high titers of replicating virions within chronic mucocutaneous lesions, and widespread use of acyclovir will continue to predispose to the development of acyclovir resistance in HIV-infected hosts. In addition, our preliminary data suggests that the emergence of resistance to foscarnet may become an increasingly clinically significant problem. Therefore, further information regarding the epidemiology, clinical virologic manifestations, and genotypic characterization of resistant HSV mutants will be critical in the devising of effective preventive and therapeutic maneuvers. A survey of 300 consecutive clinical isolates of HSV from HIV-infected patients at San Francisco General Hospital will be conducted to determine the prevalence of in vitro resistance to acyclovir. The presence of viral heterogeneity in recovered isolates will be assessed using plaque autoradiography, and correlative clinical information will assist in the elucidation of the interface between in vivo manifestations of antiviral resistance and in vitro testing results. Further characterization of a sample of 29 patient isolates utilized in the comparative assessment of the three primary methods of antiviral susceptibility testing currently in use will be performed in order to clarify differences in results, and a new method utilizing a non-radioisotopic DNA hybridization methodology will be critically assessed, modified and standardized for general usage. Sequencing of the DNA polymerase gene of foscarnet-resistant HSV mutants from 5 HIV-infected patients, unusual in their susceptibility to acyclovir, will be undertaken using paired prefoscarnet therapy isolates for comparison. Identification of the locus of DNA polymerase mutation will increase our understanding of the nucleotide binding site of the enzyme, and facilitate the future development of rapid diagnostic tests for antiviral resistance.